A bounce is a message that is returned to the server that sent it. Bounced emails are either a permanent failure to deliver the email (hard bounce) or a temporary failure to deliver the email (soft bounce), based on conditions with the recipient mail server.
A soft bounce means that the email address was valid and the email message reached the recipient’s mail server. However, it bounced back because:
The mailbox was full (the user is over their quota)
The server was down
The message was too large for the recipient’s inbox
After a soft bounce, the software will continue to try sending the email for up to 72 hours, so you may find that the total number of bounces reduces.
A hard bounce occurs when the message has been permanently rejected either because:
The email address is invalid
The email addresses doesn’t exist
In Eagle, the Bounces tab only shows hard bounces (as the soft bounces will continue to retry), which is why there is a discrepancy with these figures.